Low cost insertable type port liner

ABSTRACT

A method and apparatus for insulating the exhaust passage of an internal combustion engine is disclosed. A three-zone liner assembly is provided with an outer zone comprised of a room temperature vulcanizing silicone sleeve, an inner zone comprised of a stamped and seam welded high strength Al--Cr--steel alloy, and an intermediate zone consisting of a ceramic wool mat. The liner assembly is supported or enclosed within a mild carbon sheet metal sleeve which in turn may be bonded to the engine passage wall by use of a room-temperature-vulcanized silicone if of the insert type, or by fusion bonding during casting if of the cast-in-place type.

BACKGROUND OF THE INVENTION

With the advent of stricter governmental controls for engine emissionsand increased concern to reduce weight of passenger vehicles, therearises a need for conserving the residual heat of exhaust gases of aninternal combustion engine so that downstream equipment in a vehicleexhaust system may operate with higher efficiency and effectiveness toreduce the emission levels of the engine and conserve fuel. This needhas become quite apparent to the automotive industry and is currentlyunder intense development effort. Any solution to this problem must besimple, durable, and yet not introduce any additional problems.

Heat loss, experienced by the exhaust gases as they travel from thecombustion zone through the exhaust passage of the engine block, can beconsiderable. Such heat loss is accomplished by conduction, convectionand radiation. Minimizing heat loss within the exhaust passage isimportant for at least two principal reasons, (a) to maintain a hightemperature of the exhaust gases therein to induce oxidation, and (b) toreduce the heat loss to the surrounding coolant in the block and head soas not to permaturely dissipate an unduly large number of heat units.

The prior art has approached such problems in principally three modescomprising: (1) use of cast-in-place type liners which have been eitherof the single metal layer or single refractory element design, or dualmetal or refractory layers; (2) the use of insertable type liners whichare added independently of the fabrication of the engine housing, suchliners also being of the single layer heat resistant alloy metal designor double layer metal design or multiple layers of ceramic including airspaces or foamable paste therebetween; and (3) the use of appliedcoatings directly to the prefabricated engine housing passage walls,including asbestos and other ceramic materials. The disadvantage toemploying cast-in-place type liners to date has been principally a lackof bonding; shrinkage and solidification of the cast metal around theliner has lead to localized poor bonding and/or separation whicheventually provides for leaks and inadequate insulation. The principaldisadvantage to the insertable type liner is that they insufficientlycontrol heat transfer by not conforming closely to the wall of theexhaust passage resulting in a poorly trapped air space and a reductionin the insulating factor resulting from sealing difficulties. Coatingshave proved disadvantageous because of their fragile nature which isparticularly troublesome when the cast housing is subjected to postmechanical or chemical treatments tending to fracture or chip suchcoatings. Moreover, such coatings require multiple steps which result inincreased manufacturing costs.

SUMMARY OF THE INVENTION

A primary object of this invention is to provide a new and improvedmethod of making exhaust passage insulating liners for an automotiveengine, the method being characterized by (a) increased economy offabrication and material while providing for improved bonding of theliner to other components of the engine system, and (b) has a decreasedtotal coefficient of heat transfer from the exhaust passage wallcompared to prior art liners.

Yet still another object of this invention is to provide a low cost heatinsulating liner for the exhaust passage of an engine which liner notonly minimizes heat transfer across the total thickness of the liningassembly but also provides a low specific heat at the inner structure ofthe liner to minimize chill to the exhaust gases passing therethroughparticularly during a cold start. The inner structure shouldadditionally provide increased resistance to oxidation at hightemperatures.

Yet still another object of this invention is to provide an improvedexhaust port liner meeting the above objects and which has an extendedoperating life of at least 5000 hours and is characterized by a highresistance to erosion both from chemicals and mechanical abrasion eitherduring use or during fabrication of the engine housing.

Features pursuant to the above objects comprise (a) the use of a threezone liner wall assembly, (b) the supporting structure for the assemblyis comprised of a mild carbon steel sleeve having by weight less then0.06¢ carbon and less than 0.02% impurities, (c) an outer zoneconsisting essentially of a thin sleeve of room-temperature-curablesilicone having a thermal conductivity of about 0.008 BTU(ft.)/hr.ft².°F., (d) an intermediate zone having trapped air spacesdefined by foam or fiber wool, and (e) an innermost zone comprised of aweldable heat resistant and chemically resistant alloy consistingessentially of iron-chromium-aluminum.

SUMMARY OF THE DRAWINGS

FIG. 1 is a sectional view of a portion of an engine housingillustrating the positioning of an insertable type liner according tothe principles of this invention;

FIG. 2 is an enlarged fragment of the sectional view of the three zonedwall system of the liner displayed in FIG. 1;

FIG. 3 is a view similar to FIG. 2, but illustrating a portion of acast-in-place type liner assembly according to the principles of thisinvention.

DETAILED DESCRIPTION

The purpose of the liner of this invention is to minimize the heat lossthrough the exhaust port walls thus increasing the exhaust gastemperature to induce hydrocarbon oxidation, improve the downstreamthermal reactor and/or catalyst efficiency, reduce the heat transfer tothe engine coolant, and to all of the above by way of a low costassembly. To function as an efficient port liner, the materials and theconstruction of the liner walls must meet the following requirements forthis invention: (a) the heat transfer across the assembly wall from theexhaust gases to the cast metal must be minimized, preferably to lessthat 25% of the heat loss experienced by an unlined passage, (b) thematerials used in each zone of the assembly must be thermally stable atthe gradient temperature experienced at each respective zone, (c) theinner skin material for the liner should (i) have a very low specificheat of about 0.10 BTU/lb./°F., to minimize chill to the exhaust gasesduring cold startup operations, (ii) have low thermal mass, (iii)possess good chemical oxidation resistance and withstand thermaltemperatures up to 1600° F., and (iv) yield at least 3000 hours ofservice life in an engine exhaust environment. In addition, thesupporting sleeve for the assembly should withstand the chemical erosioncaused by the molted metal during casting if of the cast-in-place typeassembly and the exposed surfaces of the liner should withstand themechanical erosion caused by the exhaust gases or the mechanical shockand abrasion caused by shot-peening, employed during cleanup of theengine housing.

APPARATUS

To meet the above criteria, one preferred mode of the present inventionprovides for an exhaust port liner with at least three zones, theoutermost zone A is comprised of a room-temperature-curing siliconeresine, such as a solventless polysiloxane with a melting point of200°-220° F. and a thermal conductivity of about 0.008BTU.ft./hr.ft².°F. In the presence of a catalyst such as argon ormetallics, the silicone is thermoset through the condensation of thehydroxyl groups. One such compound is polymethyl siloxane silicone madeby General Electric or Dow Corning. The silicone is formed as a thinsleeve and is thermally stable at temperatures up to 200° F. which isthe temperature environment for the thin layer juxtaposed to thewater-cooled engine housing. The thickness of the silicone sleeve isabout 0.01 inch or less.

The intermediate zone B is comprised of one or more trapped air spacespreferably occupied by ceramic fiber wool or mat such as aluminumsilicate or cordierite (the latter is a ceramic consisting of magnesiumaluminum silicate 2MgO. 2Al₂ O₃.5 SiO₂, or other stable low thermalconductivity ceramic. The fiber may be employed in the mat form oncollected wool; each form serves to define numerous trapped air spacesgiving the intermediate zone a thermal conductivity value of 0.5 BTU.ft./hr.ft².°F. The ceramic is stable at temperatures of 400°-600° F.which are experienced in this zone.

The third or innermost zone C is comprised of an inner sheet metal skin,the metal consisting essentially of a low aluminum-chromium steelcontaining approximately 18% chromium, 2% or less aluminum, and theremainder iron. In some instances the alloy may contain a small amountof yttrium at about 0.5%. Such chemistry provides for a thermalconductivity of 12.5 BTU. ft./hr.ft².°F. and provides for weldability tothe mild carbon steel outer skin while at the same time providing forresistance to chemical erosion at a relatively low cost. Because theinner skin has a high strength and is not deep-drawable, fabricationmust be by stamping and subsequent welding along predetermined seams.

The supporting structure for the liner assembly, which is juxtaposed atpassage wall and encloses the assembly, is comprised of a mild carbonsheet steel designed to have a melting temperature higher than themelting temperature of a cast iron engine housing into which the lineris implanted or inserted. The cast iron should be typically of the greyiron type having a chemistry consisting of 3-4% carbon, 1-2% silicon andthe remainder Fe. For nodular iron, 0.5% or less MgO is present. Themelting temperature for such a grey cast iron is about 1150°-1200° C.and the melting temperature for the low carbon sheet steel, required forthis invention should be above 1500° C. To maintain such elevatedmelting temperature for the outer skin steel, the carbon content of thelow carbon steel should be at 0.06% or less and impurities should be0.2% or less. The steel sleeve prevents heat shorts which occur withprior art cast-in-place metal liners, since in the past the molten metalpenetrated through the liner metal by solution creating metal-to metalat heat shorts for thermal transfer.

Mounting of the three zones of the liner assembly to the supportingsleeve is promoted by welding of the inner skin to the support sleeve,as described later in connection with the method of making, therebyenveloping zones A and B. The intermediate zone B is held in place tothe inner skin C during assembly or welding by the adhesive qualities ofa silicone plastic coating which subsequently deteriorates underoperating temperature conditions of liner use. Similarly, the adhesivequalities of the outer zone provides positioning as coating duringassembly, but the integrity of later zone is maintained stablethroughout the operating life of the liner since the use temperature atthe zone A never exceeds 200° F.

Metal cost is a most important factor in the present automotive enginemarket; mild carbon steel has a current price range of about 5-10 centsper pound and it is possible to obtain supplies of low aluminum-chromiumsteel for the inner skin at a price level of about $1.40 per pound. Allother chemically resistant sheet metals are considerably more expensiveor not weldable for the purpose as stated above, or cannot withstand a1200° F. temperature gradient which is necessary for the inner skin.Thus the selection of these two metals with their accompanying physicalcharacteristics in combination serve an important economicalconsideration.

The sizing of the liner is relatively important, the outer skin A musthave a thickness of 0.01 inches or less, the intermediate zone B shouldhave a thickness in the range of 0.06-0.08 inches, the inner skin Cshould have a thickness of about 0.025-0.030 inches, and the supportingmild carbon steel sleeve should have a ply thickness of 0.015-0.018inches for an insertable type liner, but 0.045-0.06 inches for acast-in-place liner. The total assembly should have a thickness of about0.125 inches across the three zones and steel sleeve; the clearancebetween the outer surface of the steel sleeve and the passage of theengine housing containing the liner, should be 0.015-0.05 inches if theliner is of the insertable type. This latter spacing is filled by aroom-temperature-curing silicone applied as a coating before insertion.The average thermal conductivity for the steel sleeve and assembly willbe about 1.5 BTU.ft./hr.ft².°F.

In the event the engine housing containing such liner is comprised ofaluminum alloy, it will typically be an aluminum-silicon alloy having amelting temperature in the range of about 600° C. In that event thesupporting sleeve will still be preferably comprised of plain carbonsteel, although a substantially pure aluminum sheet metal having athickness of about 0.025 may also be used. For cost reasons, however,the supporting sleeve should be low carbon iron, irrespective of whethera cast-in-place or insertable type liner.

METHOD--INSERT TYPE

A preferred method of fabricating a liner of the insert type, asillustrated in FIGS. 1-3, is as follows:

1. Form a sand core to define an exhaust passage 10 in a metal casting11, the core providing for a predetermined passage configuration asshown in FIG. 1. The passage configuration is comprised of a cylinder 14and an elbow 15 providing an abrupt turn at the innermost end; the elbow15 is interrupted by a flattened shoulder 16 to provide a valve guideentrance. The core is adapted to extend from the sidewall 12 of theintended casting to the lowermost wall 13 of the intended casting, theplanes of such walls being at an angle with respect to each other ofabout 75°. Several of these cores may be employed as a cluster to definea series of exhaust passages in accordance with conventional art.

2. After having placed the core in proper position within a mold, acasting for an engine head is formed thereabout using cast iron having achemistry consisting of 3-4% carbon, 1-2% silicon and the remainderiron.

3. Male and female dies are formed to define a liner support sleeve 17.The two dies are employed to deep-draw a selected metal blank, theproduct of such deep-drawing producing a configuration conformingclosely to the configuration of the cast exhaust passage with asubstantially uniform clearance of about 0.015 inches. The supportsleeve 17 has an annular flange 17a at one end adapted to abut and fittightly against the outer sidewall 12 of the engine head; sleeve 17 hasa cylindrical channel 17b adapted to extend from the flange into theelbow of the passage 10 adjacent its entrance.

4. Employing said male and female drawing dies, a blank of mild carbonsteel having, by weight, less than 0.06% carbon and less than 0.2%impurities. The low carbon steel blank is drawn to the configuration asillustrated which extends in most cases a distance of 2-3 inches fromthe flange 17a.

5. Male and female stamping dies are defined to form an inner skin orzone C for said liner assembly. The inner skin is a metal cylinder 20adapted to nest within the outer metal support sleeve 17 and provide fora predetermined spacing therebetween of about 0.08 inches, except at theleading and trailing portions where the metal sleeve and inner skin arebrought together for joining and assembly.

6. Forming a cylinder with an open longitudinal seam 20, using thestamping dies. The cylinder of skin 20 conforms to the configuration ofthe sleeve 17 except that it is spaced inwardly said 0.08 inches. Theinner skin 20 is formed from a blank of temperature resistant lowaluminum-chromium steel. Preferably the chemistry should contain 18%chromium, 2% aluminum and the remainder iron; in some cases the additionof yttrium in an amount of about 0.5% may be desired. The seam 20 isclosed by appropriate welding.

7. The completed inner and outer skins are brought together for assemblyat the leading and trailing portions 21-22 and are spot welded together.

8. Prior to welding, a mat of ceramic fiber is implanted between theskins and held in position temporarily, particularly during welding, byuse of a room-temperature-curable silicone rubber compound. The compoundis spread on the mat prior to implantation, both on the inner as well asouter surface of the mat to define two coatings 24 and 25 (the latterconstituting the outer zone of the liner assembly); each at a thicknessof 0.01 inches maximum.

9. After the support sleeve and inner skin have been welded together,the outer surface of the support sleeve 17 is also coated with aroom-temperature curable silicone rubber compound, the coating 25 beingin the thickness range of 0.010-0.050 inches.

10. The liner assembly is then inserted into the cast exhaust passage 10so that flange 17a abuts the sidewall 12 of the casting and the siliconecompound coating 25 is in intimate contact with the walls of the passage10. Thus, the liner will be supported not only by the silicone compoundcoating throughout its longitudinal extent but also by the flange 17awhich is secured to the casting such as by bolts.

METHOD--Cast-in-place

In the event the liner assembly is desired to be of the cast-in-placetype, the fabrication method is modified so that the supporting sleeve17 has a contour and dimension such that it will be entrained by themolten metal poured therearound and act as an anchored outer skin. Thesupport sleeve, of course, will not carry any silicon coating becausethe molten metal will have an initimate metallurgical bond between thecasting and the outer skin. The support sleeve 17 will maintain itsintegrity during casting because its melting temperature (1500° C.) willbe adequately elevated beyond that of the temperature of the moltenmaterial to prevent dissolution. The molten cast iron should have achemistry consisting of standard nodular iron grade or grey iron grade,thereby providing for a melting temperature of about 1200° C. Themelting temperature of the support sleeve 17 will be greater than 1500°C. as mentioned earlier. The liner is, of course, prepared and assembledprior to being cast-in-place similar to the previous process for theinsert type, except that when it is assembled it is employed as a coreelement and the molten metal cast therearound to mutually reachtherewith and provide a tight metallurgical bond throughout the entireouter surface of sleeve 17. The positioning of the cast-in-place lineris illustrated in FIG. 3.

We claim:
 1. For use in an internal combustion engine having a housingmass enclosing a combustion zone in one portion thereof, apparatus formodifying the heat transfer characteristic through the walls of anunlined passage extending through said portion of the mass of saidengine, said passage walls defining an outer wall and an interior space,insulating means in said passage conforming substantially to the innercontour of said passage outer wall and having a layered assembly with atleast five radially distinct zones, the outermost or first zone of saidlayered assembly consisting of a room-temperature-curable siliconeplastic in the thickness range of 0.02-0.05 inches, a second zoneconsisting of a plain carbon steel sleeve containing less than 0.6%carbon and having a thickness in the range of 0.015-0.018 inches, athird zone consisting of a coating of room-temperature-curable siliconeplastic in a thickness of about 0.01 inches and having a meltingtemperature greater than 200° F., a fourth zone consisting of aninsulating wool or fiber mat in the thickness range of 0.020-0.060 inchproviding numerous enclosed trapped air spaces, and a fifth or innermostzone consisting of a temperature resistant and weldable metal alloy,said latter zone having a thickness of 0.020-0.040.